Lymphocytes and leukemia viruses: tropism and transtropism ofmurine leukemia virus

SUMMARY-The tropism of naturally occurring murine leu-kemia virus (MuLV) was investigated in short-term lymphocyte cultures. The tropism of MuLV was readily defined in fibroblast cultures, but not in lymphocyte cultures. Lymphocytes free of infectious MuLV could be infected across the tropism barrier by partially purified MuLV or by in vitro contact with MuLV-producing lymphocytes. Stimulation of lymphocytes was not required for this cross-infection and replication of MuLV. When cross-infected lymphocytes were stimulated in vitro by allo-geneic cells, they facilitated MuLV infection of ordinarily non-permissive fibroblasts. This phenomenon (transtropism) re-quired antigenically stimulated lymphocytes and was specifically associated with infection of the lymphocyte by MuLV across the tropism barrier. Thus in contrast with the resting lympho-cyte, the transformed lymphocyte acquired the ability to dis